Many types of zinc ores can be readily dissolved in sulphuric acid, either directly, or after pre-treatment, such as concentration through flotation followed by oxidation, to render the zinc more amenable to leaching. The leaching of these ores, including the pre-treatment and the removal of impurities, such as silica and iron, for disposal and the recovery of valuable metals associated with the zinc in the ore, such as copper, by precipitation using zinc dust additions, is well established.
Although there are many processes that can provide a method to dissolve the zinc from acid soluble ores and other sources, or that can generate secondary streams of dilute acidic zinc bearing solutions, none of the processes provide a versatile and cost-effective process for the recovery of zinc. The most common method for the recovery of zinc from a zinc sulphate solution is by the conventional zinc electrowinning process. It requires considerable capital investment and may not be a suitable process to install at the location where the dilute acidic zinc solution is available or where sufficient energy is unavailable or expensive. To compete commercially, an electrolytic zinc plant requires some strategic advantage. This advantage may be derived due to its large scale, low capital costs or mostly depreciated capital cost, a location close to zinc markets and with good infrastructure, labour costs that are lower than its competitors and a large labour pool from which to draw, or a low cost ample supply of electricity. Many low grade sources of zinc that can be leached to form a dilute acidic zinc sulphate solution are not available in quantities that would justify a large electrolytic zinc plant. A large labour pool or ample electricity at competitive rates for electrolysis may not be available at the site where a leachable zinc deposit is found. As well, it may be possible to separate a secondary stream of zinc-bearing solution from a process in quantities that do not justify an electrolytic zinc plant or at a location that is inappropriate to install a zinc electrolysis operation for one or more of the reasons cited.
A zinc sulphate solution can be concentrated slightly by several known methods, such as solvent extraction or ion exchange. The primary limitation of these processes is that they still require a conventional zinc electrowinning process to recover the zinc from the resulting concentrated zinc sulphate solution and to regenerate the acid that is required to strip the zinc from the solvent or ion exchange resin.
Another approach to recover zinc from the dilute zinc bearing solution is to precipitate the zinc from the solution. The recovered zinc can be sent to a zinc refinery for further processing and recovery. Zinc can be recovered from a zinc sulphate solution by adding an alkaline agent to produce zinc oxide, zinc hydroxide, or basic zinc sulphate. Zinc can also be precipitated form a zinc sulphate solution by adding a sulphide to produce zinc sulphide. Similarly, zinc can be precipitated as zinc carbonate by adding a carbon dioxide or a carbonate.
When precipitating zinc as zinc hydroxide, basic zinc sulphate or zinc oxide, usually the preferred alkaline agent is calcium oxide or hydrated lime, calcium hydroxide, because it is more readily available and less expensive than other alkaline reagents. When using calcium oxide as the alkaline agent, the resulting zinc-bearing precipitate is highly diluted by the associated gypsum which is generated by the reaction of calcium oxide and sulphate in the dilute acidic solution. Another alkaline source could be used, such as sodium hydroxide, but these are very expensive and would make the recovery of zinc uneconomic. For these reasons the recovery of zinc by precipitation with calcium oxide to produce zinc hydroxide, basic zinc sulphate, or zinc oxide is usually associated with an electrolytic zinc plant where the zinc in the resulting precipitate is re-leached to separate it from the gypsum without the expense of drying and transporting such a low grade zinc intermediate product.
A higher grade and transportable zinc sulphide precipitate can be produced from a dilute zinc sulphate solution by introducing a sulphide or carbon oxide. These sulphides can be either purchased, such as sodium hydrosulphide, or generated on site, such as hydrogen sulphide, which is usually generated by bacteriological processes. In all cases, the sulphide source is expensive and, if other sulphide consuming metals are present in the dilute zinc sulphide solution, they will consume sulfidizing reagent and render the process uneconomic. Similarly, the reagents required to produce a zinc carbonate precipitate are very expensive so that it would not be economic to recover the zinc by such a process.
As well as a solution produced by the leaching of zinc ores, there are many sources of dissolved zinc generated by secondary process or effluent streams from processes. The most common is the production of a dilute acidic zinc sulphate bearing solution generated by acid mine drainage, or the contaminated acidic water which drains form the tailings impoundment areas of zinc mines. This acidic water is usually collected and sent to on-site effluent treatment plants where it is neutralised with calcium oxide to remove the zinc and other heavy metals. These effluent streams may also be treated with a sulphide, such as hydrogen sulphide, to recover a saleable zinc sulphide product.
U.S. Pat. No. 6,726,889 B2 describes a process to overcome the fact that the resulting zinc precipitate is very low grade when the zinc is removed from the dilute sulphate solution by the addition of calcium oxide. This process involves the use of conditions that favour the production of zinc oxide, rather than basic zinc sulphate during the precipitation of the zinc form the dilute zinc sulphate solution. These conditions include a temperature of 95° C., which is some 30° C. higher than the temperature required for the precipitation of basic zinc sulphate. Also the pH required to favour the formation of zinc oxide is approximately pH 10 while virtually all of the zinc can be precipitated from a dilute sulphate solution at a pH of approximately pH 6.5. The precipitation reaction for basic zinc sulphate requires less neutralising agent than is required for the precipitation of zinc oxide. The additional cost for heating and neutralising the dilute solution beyond what would be required for precipitating all of the zinc as zinc oxide rather than basic zinc sulphate is substantial.
Therefore, it is desirable to develop a process for the production of basic zinc sulphate from a dilute zinc sulphate solution to produce a relatively pure basic zinc sulphate and avoid the dilution of the zinc precipitate by the co-precipitated gypsum.